This review seeks to evaluate the role of low oxidation state transition metal clusters (metal carbonyl clusters) in catalysis. The review commences with a discussion of the cluster surface analogy and then proceeds to discuss whether catalysis takes place by intact molecular clusters or mononuclear fragments; both these topic have been fiercely contested in the literature. Next, the application of clusters in organic transformations is briefly reviewed with an emphasis given to reactions that cannot be conducted using other catalysts (or pre-catalysts); areas for future studies are also highlighted. The latest developments in cluster immobilisation using alternative solvents such as water, ionic liquids and supercritical fluids, and the use of clusters as precursors to heterogeneous nanocluster catalysts are reviewed. Finally, the possibility of using materials at the cluster-colloid interface in catalytic: applications is discussed.(C) 2004 Elsevier B.V. All rights reserved.